So-called boding technology has been used for several years in the semiconductor industry. Bonding technology allows joining of two or more substrates which are generally aligned very precisely to one another. In most cases this joining takes place permanently, therefore irreversibly, which means that the separation of the two substrates after the bonding process is no longer possible without their destruction or at least their partial destruction. When the substrates are joined it is shown that there are different chemical and physical mechanisms which cause a permanent connection. Nonmetallic surfaces are especially interesting. In nonmetallic surfaces the formation of a so-called prebond occurs by pure contact-making.
This spontaneously forming, reversible connection of the two substrates which is caused via surface effects is called a prebond in order to distinguish it from the later actual bond which is no longer separable, therefore is irreversible, and which is caused by an additional heat treatment. The prebond which has been produced in this way is still characterized by a strength which should not be underestimated. Although wafers which have been joined to one another in this way must still be heat-treated at higher temperatures for a permanent bond, the strength of the prebond is sufficient to fix the two substrates until the next process step. The prebond is an extremely useful means for preliminary fixing of two substrates, mainly after an alignment process, since the two substrates after the alignment process should no longer move toward one another. The prebond should be based mainly on van der Waals forces which are present due to permanent and induced dipoles on the surface of the substrate. Since the van der Waals forces are very weak, a correspondingly high contact area is necessary so that a notable adhesion action occurs between the substrates. Unpolished solid surfaces however do not make optimum contact at correspondingly high roughness. In the case of pure solid contact, prebonds therefore arise mainly between the very flat polished substrate surfaces. At room temperatures, under certain circumstances already isolated covalent bonds can also form between the substrates surfaces, even without additional application of temperature and/or force to the substrates. The number of covalent bonds which have formed at room temperature should however be negligibly small.
Mainly the use of liquids could increase a corresponding adhesion action between substrates. On the one hand, the liquid equalizes the unevenness on the surfaces of the substrate and itself preferably forms even permanent dipoles. A pronounced prebond capacity is established mainly on nonmetallic surfaces. Semiconductors such as silicon, ceramics, here mainly oxides, metal oxides, which are polished and extremely flat, upon making contact show a corresponding behavior.
For nonmetallic surfaces, therefore surfaces which show a predominantly covalent bond character, such as for example Si, SiO2, etc., a previously applied liquid film can even contribute to strengthening of the permanent bond by covalent bonds which arise during heat treatment. The nonmetallic surfaces are subjected to heat treatment after a prebond. The thermal activation produces covalent bonds between the surfaces and thus produces an irreversible connection. Thus single-crystalline, highly precisely cut and ground silicon wafers are welded to one another mainly by the formation of covalent bonds between the silicon atoms. If a silicon oxide is on a silicon wafer, mainly covalent silicon oxide bonds and/or oxide-oxide bonds form. It has been shown that the use of very thin liquid layers, generally of water, causes or at least improves the formation of covalent bonds between the surfaces. The liquid layers are only a few nanometers thick or even consist only of a single monolayer of the liquid. The liquid layers thus improve not necessarily only the prebond behavior, but also contribute significantly to the formation of covalent connections. The reason, in the case of water, lies mainly in making available oxygen as a connection atom between the atoms of the substrate surfaces which are to be bonded to one another. The binding energy between the hydrogen and the oxygen of a water molecule is low enough to be broken with the applied energy. New reaction partners for the oxygen are then mainly the atoms of the substrate surfaces. In any case it should be mentioned that there are surfaces in which these processes in which atoms of the liquid participate directly in the permanent bond process of the substrates surfaces need not necessarily occur.
The bond process for pure metal surfaces runs quite differently. Since metals behave chemically and physically completely differently due to their metallic bond nature, a completely different bond strategy is required. Metals are bonded to one another mainly at higher temperatures and generally under very high pressure. The high temperatures lead to intensified diffusion along the surfaces and/or the grain boundaries and/or the volume. Due to the increased mobility of the atoms, different physical and chemical effects occur which lead to a welding of the two surfaces. The disadvantage in these metal bonds therefore consists mainly in the use of very high temperatures and pressures to ensure a joining of the two substrates at all. In the overwhelming number of eases pure metal surfaces will not be found beforehand. Almost all metals except for very inert metals such as Pt, Au and Ag are coated in the atmosphere with an oxide layer, even if only very thin. This oxide layer is sufficient to produce a prebond even between the metal surfaces which are covered with a very thin oxide layer. In any case this oxide layer is in turn unwanted if the intention is to bond two metals directly to one another, for example to join two conductive contacts to one another.
Heat treatment of the substrates dictates correspondingly long heat-up and cooling times. The high temperatures can moreover lead to disruptions in functional units such as for example microchips and mainly in memory chips and can damage them to the point of not being usable.
Furthermore, substrates with corresponding surfaces can be aligned to one another prior to the actual bond step. This alignment, once carried out, should no longer be destroyed as far as the final, therefore permanent, bond process. In any case, mainly at higher temperatures, due to the different coefficients of thermal expansion of different materials and the resulting thermal stress, generally a shift of different component regions of the substrates to one another takes place. In the worst case the two substrates which are to be joined to one another are comprised of two different materials with different coefficients of thermal expansion. These shifts are the greater, the greater the difference of the coefficients of thermal expansion of the different materials.